feat(tvix/castore): bump [Directory,File]Node size to u64

Having more than 4GiB files is quite possible (think about the NixOS
graphical installer, and an uncompressed iso of it).

No wire format changes.

Change-Id: Ia78a07e4c554e91b93c5b9f8533266e4bd7f22b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9950
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/castore/src/tests/import.rs

@@ -56,7 +56,7 @@ async fn single_file() {
         proto::node::Node::File(proto::FileNode {
             name: "root".into(),
             digest: HELLOWORLD_BLOB_DIGEST.clone().into(),
-            size: HELLOWORLD_BLOB_CONTENTS.len() as u32,
+            size: HELLOWORLD_BLOB_CONTENTS.len() as u64,
             executable: false,
         }),
         root_node,